Reflective photomask and reflection-type mask blank

Abstract

A reflective photomask includes a substrate with a substrate layer of a low thermal expansion material. The substrate layer includes a main portion of a first structural configuration and an auxiliary portion of a second structural configuration of the low thermal expansion material. The auxiliary portion is formed in a frame section surrounding a pattern section of the substrate. A multilayer mirror is formed on a first surface of the substrate. A reflectivity of the multilayer mirror is at least 50% at an exposure wavelength below 15 nm. A frame trench extending through the multilayer mirror exposes the substrate in the frame section. The auxiliary portion may include scatter centers for out-of-band radiation.

The invention claimed is: 
     
       1. A reflective photomask, comprising
 a substrate comprising a substrate layer of a low thermal expansion material, the substrate layer comprising a main portion of a first structural configuration and an auxiliary portion of a second structural configuration of the low thermal expansion material, wherein the auxiliary portion is formed in a frame section surrounding a pattern section of the substrate; and 
 a multilayer mirror formed on a first surface of the substrate, wherein at an exposure wavelength below 15 nm a reflectivity of the multilayer mirror is at least 50% and wherein a frame trench extending through the multilayer mirror exposes the substrate in the frame section. 
 
     
     
       2. The reflective photomask of  claim 1 , wherein
 a thermal expansion coefficient of the main portion of the low thermal expansion material is less than 1 ppm/K. 
 
     
     
       3. The reflective photomask of  claim 1 , wherein
 the auxiliary portion comprises scatter centres for radiation in an out-of-band wavelength range from 100 nm to 300 nm. 
 
     
     
       4. The reflective photomask of  claim 3 , wherein
 in the out-of-band wavelength range a mean reflectivity of the auxiliary portion is at most 50% of a mean reflectivity in the main portion. 
 
     
     
       5. The reflective photomask of  claim 1 , further comprising
 an absorber stack formed on the multilayer mirror opposite to the substrate, wherein an absorbance of the absorber stack at the main exposure wavelength is at least 50% and pattern trenches expose portions of the multilayer mirror in the pattern section. 
 
     
     
       6. The reflective photomask of  claim 1 , wherein
 the frame trench forms a continuous frame surrounding the pattern section. 
 
     
     
       7. The reflective photomask of  claim 1 , wherein
 an effective index of refraction in the frame section differs by at least 10% from an effective index of refraction in the main portion. 
 
     
     
       8. The reflective photomask of  claim 1 , wherein
 the main portion is amorphous. 
 
     
     
       9. The reflective photomask of  claim 1 , wherein
 the auxiliary portion is in a distance of at least 5 nm to the first surface. 
 
     
     
       10. The reflective photomask of  claim 1 , wherein
 the auxiliary portion is formed by irradiation with laser pulses of a wavelength of 532 nm. 
 
     
     
       11. The reflective photomask of  claim 1 , wherein
 the exposure wavelength is in a range from 6 nm to 14 nm. 
 
     
     
       12. The reflective photomask of  claim 1 , wherein
 the auxiliary portion forms a continuous structure along the frame section. 
 
     
     
       13. The reflective photomask of  claim 1 , wherein
 the auxiliary portion comprises isolated sections lined up along the frame section. 
 
     
     
       14. The reflective photomask of  claim 1 , wherein
 the auxiliary portion comprises scatter centres for radiation in an out-of-band wavelength range from 100 nm to 300 nm, horizontal dimensions of a single scatter centre parallel to main surfaces are in a range from 0.5 μm to 2 μm, and a mean centre-to-centre distance between neighbouring scatter centres in the same horizontal plane is in a range from twice to twenty times the horizontal dimensions. 
 
     
     
       15. A reflection-type mask blank, comprising
 a substrate comprising a substrate layer of a low thermal expansion material, the substrate layer comprising a main portion of a first structural configuration and an auxiliary portion of a second structural configuration of the low thermal expansion material, wherein the auxiliary portion is formed in a frame section surrounding a pattern section of the substrate; and 
 a multilayer mirror formed on a first surface of the substrate, wherein at an exposure wavelength below 15 nm a reflectivity of the multilayer mirror is at least 50%. 
 
     
     
       16. The reflection-type mask blank of  claim 15 , wherein
 a thermal expansion coefficient of the main portion of the low thermal expansion material is less than 1 ppm/K. 
 
     
     
       17. The reflection-type mask blank of  claim 15 , wherein
 the auxiliary portion comprises scatter centres for radiation in an out-of-band wavelength range from 100 nm to 300 nm. 
 
     
     
       18. The reflection-type mask blank of  claim 17 , wherein
 in the out-of-band wavelength range a mean reflectivity of the auxiliary portion is at most 50% of a mean reflectivity in the main portion. 
 
     
     
       19. The reflection-type mask blank of  claim 15 , further comprising
 an absorber stack formed on a surface of the multilayer mirror opposite to the substrate, wherein an absorbance of the absorber stack at the main exposure wavelength is at least 50%. 
 
     
     
       20. The reflection-type mask blank of  claim 15 , wherein
 the frame section forms a continuous frame. 
 
     
     
       21. The reflection-type mask blank of  claim 15 , wherein
 an effective index of refraction in the frame section differs by at least 10% from an effective index of refraction in the main portion. 
 
     
     
       22. The reflection-type mask blank of  claim 15 , wherein
 the auxiliary portion forms a continuous structure along the frame section. 
 
     
     
       23. The reflection-type mask blank of  claim 15 , wherein
 the auxiliary portion comprises isolated sections lined up along the frame section. 
 
     
     
       24. The reflection-type mask blank of  claim 15 , wherein
 the auxiliary portion comprises scatter centres for radiation in an out-of-band wavelength range from 100 nm to 300 nm, horizontal dimensions of a single scatter centre parallel to main surfaces are in a range from 0.5 μm to 2 μm, and a mean centre-to-centre distance between neighbouring scatter centres in the same horizontal plane is in a range from twice to twenty times the horizontal dimensions.